External Force Observer for Small- and Medium-Sized Humanoid Robots

Abstract

External force observer for humanoid robots has been widely studied in the literature. However, most of the proposed approaches generally rely on information from six-axis force/torque sensors, which the small or medium-sized humanoid robots usually do not have. As a result, those approaches cannot be applied to this category of humanoid robots, which are widely used nowadays in education or research. In this paper, we propose a Kalman filter-based observer to estimate the three components of an external force applied in any direction and at an arbitrary point of the robot’s structure. The observer is simple to implement and can easily run in real time using the embedded processor of a small or medium-sized humanoid robot such as Nao or Darwin-OP. Moreover, the observer does not require any changes to the robot’s hardware, as it only uses measurements from the available force-sensing resistors (FSR) inserted under the feet of the humanoid robot and from the robot’s inertial measurement unit (IMU). The proposed observer was extensively validated on a Nao humanoid robot in both cases of standing still or walking while an external force was applied to the robot. In the conducted experiments, the observer successfully estimated the external force within a reasonable margin of error. Moreover, the experimental data and the MATLAB and C++/ROS implementations of the proposed observer are available as an open source package. https://goo.gl/VkhejY.

Online Gain Adaptation of Whole-Body Control for Legged Robots with Unknown Disturbances

“…For humanoid robotic applications, an external force/moment can be estimated based on an extended Kalman filter using whole-body dynamics, force sensor on the feet, and an IMU Benallegue et al (2018). In some cases, external forces can be measured using force-sensing resistors Hawley et al (2019) on the feet. We can also deploy adaptive control approaches for stabilizing the robots without direct estimation of the external forces.…”

Sensor Observability Index: Evaluating Sensor Alignment for Task-Space Observability in Robotic Manipulators

“…6-axis force-torque sensors and 3axis accelerometers, automatically observes all possible taskspace axes and renders the issue of task-space observability trivial, these advanced sensors may not always be available. For example, 6-axis force-torque sensors are not available for many lower-cost systems given their cost or their size makes them infeasible to mount, and alternative methods must be used [13]. Additionally, interactions with the robot body may not be observed if the sensor is mounted at the end effector.…”

How Heavy Is It? Humanoid Robot Estimating Physical Properties of Unknown Objects Without Force/Torque Sensors

“…Since commercial F/T sensors are often heavy and expensive, they are not commonly equipped on smaller-sized humanoid robots. Therefore, it is still challenging for smallersized humanoid robots to estimate inertial properties such as mass and center of mass (CoM) location of unknown objects [7], [8], [9]. Despite lacking direct F/T sensing ability, many smaller-sized robots possess other sensors mounted on each joint, such as encoders and electric current sensors [10], [11], [12].…”